We wish to explore the role that a transcription factor, termed hXBP-1, plays in processes of connective tissue development and disease. Human X Box Binding Protein 1 (hXBP-1) is a member of the basic region-leucine zipper class of transcription factors, which binds to cyclic AMP and TPA response elements in the promoter region of several genes. Examples of genes whose promoters contain such motifs are osteopontin, osteocalcin, TIMP, and the cytokine interleukin 6 (IL). Our data show that hXBP-1 is expressed in tissues undergoing ossification during both intramembranous and endochondral bone formation: in the mandible, calvaria, clavicle, vertebrae, ribs and long bones (1). The tissue distribution of hXBP-1 mRNA in a 14.5 day old mouse embryo is strikingly reminiscent of that of TIMP at the same stage of development (1). In Aim 1, we will directly test the hypothesis that hXBP-1 regulates TIMP gene transcription by binding studies in EMSA, and by transactivation and antisense experiments in synovial fibroblast cell lines. In Aim 2, the expression of hXBP-1 relative to TIMP and other connective tissue genes will be assessed in model organ cultures of bone and cartilage development. In Aim 3, the functional role of hXBP-1 in connective tissue development in vivo will be examined by the production of transgenic lines of mice which have sustained a loss of function mutation of hXBP-1 secondary to targeted gene disruption. In Aim 4, the role of hXBP-1 in regulating IL-6 expression in the rheumatoid joint and its interaction with the NF-IL-6 transcription factor will be explored by in situ hybridization studies and by transactivation and antisense experiments.